1. Field of the Invention
The present invention relates to selective catalytic reduction, and more particularly to methods for controlling and diagnosing such catalytic reduction as well as for controlling internal combustion diesel engines attached to such catalyst systems.
2. Background of the Invention
Diesel internal combustion engines typically operate predominantly lean of stoichiometry. As such, various emission control devices have been used to reduce NOx emissions generated by the engine operation. One such device is a lean NOx catalyst. One example of such a catalyst is a selective catalytic reduction (SCR) catalyst that utilizes a reductant, such as ammonia or urea, to reduce NOx in an oxygen rich atmosphere. Other lean NOx catalysts utilize different reductant, such as diesel fuel or other hydrocarbons, present in the exhaust gas.
One method for using such a catalyst is described in U.S. Pat. No. 5,891,409. Here, an upstream oxidative catalyst is used to convert NO into NO2 in the present of oxygen. Then, a downstream catalyst converts the NO2 to N2, CO2, and H2O in the presence of an injected hydrocarbon.
The inventors of the present invention have recognized a disadvantage with such a system. In particular, the present inventors have recognized that such catalyst systems can be susceptible to sulfur degradation. In other words, sulfur contained in fuel or engine oil can reach the exhaust catalyst and cause degradation of the active components of the catalyst. Such degradation can cause increased emissions. Further, this sulfur degradation is affected by the ratio of NO to NO2 entering the catalyst. In other words, when operating an SCR catalyst downstream of an oxidation catalyst, a mixture of substantially all NO2 can result in rapid sulfur degradation, thereby giving degraded emissions performance.